1. Field of the Invention
The present invention relates to a method for controlling the concentration of Si doped slightly in a Gap single crystal layer which is grown by means of the liquid phase epitaxial growth technique (hereinafter for short referred to as GaP liquid phase epitaxial growth layer, GaP single crystal layer or GaP layer) and, more particularly, relates to a method for controlling the very low Si impurity concentration to be a desired value with the help of an analytical method for determining simply the Si concentration by a photoluminescence spectroscopic analysis at room temperature.
2. Description of the Prior Art
Light emitting devices such as light emitting diodes and the like are generally obtained by forming a plurality of semiconductor layers an a semiconductor substrate to prepare a multi-layered semiconductor substrate having pn junction and then making it into devices for the practical use. Among the devices, Green light emitting diodes can be obtained by using a light emitting device substrate prepared by forming one or more layers of n-type GaP single crystal or of p-type GaP single crystal in succession on an n-type GaP single crystal base substrate.
Since GaP is an indirect semiconductor, the light emitting output of the device made from GaP is extremely low only with pn junction formed therein. Nitrogen (N), which functions as light emitting centers, is incorporated to a n-type GaP single crystal layer in the vicinity of the pn junction in order to enhance the green light emission. The light emitting diode fabricating from the GaP light emitting device substrate which has the Nitrogen-doped n-type GaP single crystal layer as described above, emits a yellowish-green light with the peak at a wave length of about 567 nm.
FIG. 1 shows an example of the cross-sectional structure of a GaP light emitting device substrate which emits a green light. The light emitting device substrate has an n-type GaP buffer layer 2, an n-type GaP layer 3, a Nitrogen-doped n-type GaP layer 4 and a p-type GaP layer 5 grown respectively in the written order on an n-type GaP single crystal substrate 1.
A technique for sequential growth a GaP single crystal layer on a GaP single crystal substrate 1 is the liquid phase epitaxial growth technique, which, in general, is, further, classified into two techniques called as Melt-back and Non melt-back.
In the Non melt-back liquid phase epitaxial growth technique, for example, a Ga solution is prepared by dissolving GaP polycrystal in a molten Ga at 1060.degree. C. is placed on a GaP single crystal substrate 1 and then both the solution and the GaP substrate are gradually cooled to make GaP in the Ga solution precipitate, thus a GaP single crystal layer being grown on the GaP substrate.
In the Melt-back liquid phase epitaxial growth technique, for example, after a molten Ga is placed on a GaP single crystal substrate, the temperature of them is raised to 1060.degree. C. to prepare a Ga solution by dissolving the upper portion of the GaP substrate into the molten GaP, then both Ga solution and the GaP substrate are gradually cooled to make GaP in the Ga solution precipitate, thus a GaP single crystal layer being grown on the GaP substrate.
There has been experienced a recent great progress in technologies relating to GaP Green light emitting diodes and consequently the diodes with higher light emitting output have been developed every year. Along with this trend toward higher light emitting output, the application sphere of the GaP light emitting diode has spread to a wider range. And for the further expansion of the application sphere, development of the GaP light emitting diodes with even higher light emitting output is strongly desired.
In order to prepare a light emitting device substrate specialized for fabricating light emitting devices with higher light emitting output, a method is generally known that a multi-layered GaP substrate is prepared in advance and then the melt-back technique is applied in successive growth of a GaP single crystal layer(s) thereon, as disclosed in, for example, Japanese examined patent application HEI 2-18319.
According to this method, the multi-layered GaP substrate is previously prepared by forming an n-type GaP buffer layer(s) 2 is first grown on an n-type GaP single crystal substrate 1, and in the next stage the melt-back technique is applied to the multi-layered GaP substrate in which the upper portion of the n-type GaP buffer layer 2 thereof is dissolved in a molten Ga, then dissolved GaP dissolved in a Ga solution is inversely deposited back to form sequentially an n-type GaP layer 3, a Nitrogen-doped n-type GaP layer 4 and lastly a p-type GaP layer 5 in the written order on the multi-layered GaP substrate.
In the above mentioned technique, a trace amount of Si has been frequently incorporated into GaP liquid phase epitaxial growth layers in order to improve on crystallinity of the layers as well as an n-type dopant.
When Si is used as a dopant in the GaP liquid phase epitaxial growth, it is very difficult to stabilize the concentration of Si in GaP epitaxial layers in terms of a level due to chemical properties of Si. Especially, in practical production of GaP light emitting device substrates, the stabilization of the Si concentration becomes still worse and thereby it will be extremely difficult that a Ga solution, which is a starting stuff, is used repeatedly through a plurality of runs, which is to realize mass production together with promotion of work efficiency from the repetition of the use and to reduce the cost from material economy due to again the repetition of the use.
The Si dissolved in a Ga solution is very likely oxidized and further a growth system, designed for the above mentioned liquid phase epitaxial growth, is constructed with major parts of quartz exposed directly to the ambient gas therein, which are thought to be a serious source of Si contamination in the Ga solution during the epitaxial growth. The above mentioned conditions in the liquid phase epitaxial growth will well explain the traditional unstableness of the Si concentration in GaP single crystal layers formed by the liquid phase epitaxial growth technique.
Si is well known to have an improving effect on crystallinity of the GaP single crystal layer obtained by means of the liquid phase epitaxial growth technique and it has been understood that Si should be preferably incorporated into the above-mentioned n-type GaP buffer layer 2 in the range of 2.times.10.sup.16 to 2.times.10.sup.17 atoms/cm.sup.3, taking into account the electric and emission characteristics of the GaP light emitting diode.
Such improving effect of Si on crystallinity is automatically makes an improving effect of Si on the emission characteristics of the GaP light emitting diode, but the advantageous use of Si doping has traditionally never been materialized in a practical liquid phase epitaxial growth of GaP single crystal layers due to the unstableness of a Si concentration. When the Si concentration in the Gap buffer layer(s) is lowered under 2.times.10.sup.16 atoms/cm.sup.3, a operating forward voltage of the light emitting diode fabricated therefrom is adversely increased due to the shortage of the carrier concentration therein. Also when the Si concentration exceeds to the contrary, that is, 2.times.10.sup.17 atoms/cm.sup.3, the light emitting output is adversely decreased.
In addition to those problems above mentioned, the traditional GaP liquid phase epitaxial growth technique has a further problem in that it was difficult simply and accurately to determine the Si concentration in a Ga solution. Therefore, in the past, there has been great demand for a method to directly determine simply the Si concentration in a Ga solution or to measure simply the Si concentration in a GaP liquid phase epitaxial growth layer.
There is known a secondary ion mass spectrometry as one of the analytical methods for evaluating trace amounts of impurities in materials with such levels thereof as of semiconductors. The secondary ion spectrometry analysis is quite an effective analytical method especially determine trace amounts of impurities in semiconductors, but it is destructive measurement in nature.
As compared with the secondary ion spectrometry analysis, a photoluminescence spectroscopic analysis provides a better technique in evaluation of a very low Si concentration in semiconductor materials without the above mentioned defect accompanying with the secondary ion spectrometry analysis. In the mean time, it has been a general understanding that the photoluminescence spectroscopic analysis requires to cool a crystal specimen for measurement down to cryogenic low temperatures, that is, the boiling temperature of liquid Nitrogen: 77K or that of liquid Helium: 4K in order to enable minute spectral structures by suppressing thermal vibration of electrons. Therefore, it has been rather an established understanding that the photoluminescence spectroscopic analysis is theoretically not allowed to be used at room temperature especially for a microanalysis of trace impurities in semiconductor materials and, as a result, almost no one has tried a practical use of the photoluminescence spectroscopic analysis at room temperature for the microanalysis of semiconductor materials.